1. Field of the Invention
The present invention relates to an ink jet recording method using an ink jet system in which bubbles are developed in liquid for recording or ink by using thermal energy generated by electric current feed so as to eject the ink and more particularly to an ink jet recording method which consumes very little electricity in the whole circuit and results in very little losses of electric power, and which has a good responsibility to an input signal.
2. Description of the Prior Art
The ink jet system has become of major interest lately, because it can print out at high speed and high density and because it is suitable to a color and/or compact system. In this system, there is a heating portion which allows heat to act on the liquid in order to eject liquid for recording such as ink by using thermal energy. The heating portion includes a heating resistor, and is similar in construction to a so-called conventional thermal head.
However, the ink jet system is very different from the conventional thermal head in the following points. First, the heating portion directly contacts the ink. Second, the heating portion is subjected to mechanical shock by cavitation erosion due to the repetition of the development and collapse of the bubbles. Third, the heating portion is placed under severe conditions of the repetition of sudden elevation or sudden drop in temperature around 1000.degree. C. within a short period from zero point and several micro seconds to several micro seconds.
In the conventional ink jet recording head, a first layer is formed on the heating resistor of the heating portion, the first layer consisting of materials such as SiO.sub.2, SiC, Si.sub.3 N.sub.4 and so on. The first layer serves as the electrical insulation for the heating resistor and prevents it from the oxidation thereof. A second layer is formed on the first layer and consists of a material such as Ta. Generally, the conventional ink jet recording head has the heating resistor of the heating portion which is protected by the above layers from use environment.
On the other hand, Ta.sub.2 O.sub.5 is generally used as a material of a wear resistant layer of the thermal head, but it does not always have resistivity against the cavitation erosion. As disclosed in, for example, Japanese Patent Application Publication No. 43315/1984, materials such as Ta, Ti and alloys including them are conventionally used as having strong resistivity against the cavitation erosion. The publication relates to a liquid injection recording head. It has been desired to allow thermal energy to act on the ink as efficiently and as fast as possible in order to alleviate the burden on the input signal and to reduce power consumption in the heating portion. Therefore, besides the recording head having such a protective coat, a different type of recording head having a structure such that a heating resistor directly contacts an ink (hereinafter abbreviated as a passivation free type) is proposed in Japanese Patent Application Publication No. 126462/1980. This type of the recording head is superior to the former in thermal efficiency. However, the heating resistor of the passivation free type recording head is exposed to not only the cavitation erosion and the sudden elevation and drop in temperature but also to the electrochemical reaction which is caused by passing a current through an ink having electrical conductivity.
In order to solve the above problems, a variety of metals, alloys, metallic compounds, cermets, in addition to Ta.sub.2 N and RuO.sub.2 are known as a material for the heating resistor of the conventional recording head. However, any materials described above do not have enough durability and stability to meet the necessary requirements. Ta-based alloy is proposed as a material of a heating resistor for the passivation free ink jet recording head, for example, in Japanese Patent Application Laying-open No. 96971/1984.
The above described ink jet recording head having the protective coat is available for practical use in consideration of the durability and the change of resistance. However, it is very difficult to avoid completely the occurrence of defects during the formation of the protective coat. These defects become the major factor that drops yield in mass production. Recently, with increasing requirements for high speed recording and high density of information to be recorded, and of increasing the number of nozzles per recording head, these problems become greater.
When the efficiency of thermal conductivity from the heating resistor to the ink is low, the power consumption increases as a whole, and the change in temperature of the whole head becomes greater on driving. The change of temperature of the head causes the change of volume of the ejected liquid, thus producing unevenness of density on a recorded image. In other words, when the volume of ejected liquid becomes larger, the density of pixels on a medium becomes higher. Conversely, the volume of ejected liquid becomes smaller, the density of pixels on a medium becomes lower.
Further, when increasing the number of ejections per unit time in order to record at a high speed, the power consumption at the head increases, and the unevenness of image density becomes more remarkable. This is one of the problems to be solved, because it goes against the requirement for high quality of the recorded image.
In order to solve such problems, it is desired to obtain an ink jet recording head that is practical as a head which is useful in an ink jet recording method, in which the heating resistor thereof directly contacts the ink, and in which the thermal efficiency in the heating portion is superior to the conventional ones and is independent from the defects of the protective coat.
As described above, in the passivation free type ink jet recording head, the heating resistor is exposed to not only the cavitation erosion and the sudden elevation and drop in temperature, but also the electrochemical reaction. In the conventional heating resistor, which consists of materials such as Ta.sub.2 N, RuO.sub.2 or HfB.sub.2, there are problems in durability such that it is easily mechanically broken, corroded or resolved. Materials having resistivity against the cavitation erosion described in the Japanese Patent Application Publication No. 43315/1984 can be effective only when they are used as the protective coat described above. However, they do not have enough durability when used as a material for a heating resistor for the passivation free type of the ink jet recording head. The stability of ejection of ink is essential to record at a high level of definition and high quality. Therefore, it is desirable that the resistance variation of the heating resistor is small, preferably less than 5% in practice use. When Ta-based alloy as described in Japanese Patent Application Laid-Open No. 96971/1984, for example, is used as a heating resistor of the passivation free type recording head, the alloy has relatively good durability in that the heating resistor does not break.
However, Ta or Ta--Al alloy varies its value of resistance to the extent of 7 to 10% during the repetition of the development and collapse of bubbles, thus such alloys are not satisfactory in practice use.
In addition, the ratio M of the bubbling threshold voltage (Vth) to the applied pulse voltage (Vbreak) at which the resistor may break is in the range of from 1.3 to 1.4, and thus the thermal stability of them is not so good, and there is a problem that the life of resistor greatly decreases by only a small amount of increase of a driving voltage (Vop).
As described above, when the passivation free type heating resistor is formed by any one of the conventional materials, none of such materials satisfy all of mechanical durability against the cavitation erosion, electrochemical stability, stability of resistance, heat resisting oxidation, heat resisting smelting and heat resisting shock.
The inventors found that the alloy which has one of Ta, Ir, or Al as a principal component is superior as a heating element of a passivation free type of an ink jet system. For example, Japanese Patent Application No. 503976/1990 (WO 90/09887) discloses Al--Ta--Ir alloy as materials for a heating resistor. Japanese Patent Application No. 503977/1990 (WO 90/09888) discloses Ta--Ir alloy as materials for a heating resistor. A passivation free type ink jet recording head having high durability may be prepared by these alloy materials.
On the one hand, it is however necessary to lower the cost of an integrated circuit or IC for driving and to reduce the power loss in a line. In order to achieve the above object, it is desired that an ink jet recording head is driven at a high voltage and at a low current by using the heating resistor with high resistance. In such a case, since the driving voltage is high, the ink jet recording head is exposed to more severe conditions. It has been found that sufficient durability of the alloy material cannot be obtained under such a severe condition even if the above alloy material is used.
With respect to the ink, if non-aqueous ink having a small electric conductivity can be used, it is possible to reduce the above electrochemical reaction. An example of the non-aqueous ink is one in which an oil soluble dye is dissolved in a liquid medium including an organic solvent as a main component.
However, in general, there is a problem in that an oil soluble dye is inferior to a water system in solubility and stability in the liquid medium component, wear-resistance and light-resistance, and especially safety and so on.